In order to create safe, navigable depths in sea and river ports and channels, periodic maintenance dredging must be undertaken. The size of these operations is huge; in the United States alone the annual volume of dredged silt exceeds 300 million cubic yards, or 270 million cubic meters.
Known processes generally include three operations: digging, transportation and disposal of soil. Known processes employ either suction (e.g. hopper, cutter-head and dustpan) or mechanical (e.g. dipper, ladder and clam shell) digging techniques. The dredgers alone, or with the aid of barges, forced pipelines, and conveyers, are able to transport the dredged material to either an ungerwater or upland (on shore) disposal site. (Souder P. S. et al, Dredged Material Transport Systems for Inland Disposal and/or Productive Use Concepts, TRxD--78-28, 1978, U.S. Army Experiments Station, CE Vicksburg, Miss.)
Because the entire dredging process spans over a vast space, the surrounding environment is often harmed during dredging, especially during the transportation and disposal stages. Though there are several environments adversely affected by the dredging process, the two predominant are the sea water and the sea bottom.
While the dredged soil is being transported to the disposal site, much of the material is released into the water and ultimately increases the surrounding turbidity. The soil does more harm than to simply make the water more cloudy; it also harms many microorganisms which are particularly sensitive to changes in the environment.
Released soil affects the sea bottom even more severely. The released soil blankets much of the floor adjacent the dredging site and removal route and depresses many kinds of biota, including both plant and animal life.
While underwater and upland disposal technologies both can harm the environment in the above-stated manner, they do so differently. With under water disposal, the soil is transported in the holds of barges or in hopper dredges During this process, about four percent of the total soil volume is inadvertently released into the water. (Tavolard, J. F. 1984. "A Sediment Budget Study of Clamshell Dredging and Ocean Disposal Activities in new York Bight, "Environmental Geology and Water Science 6#3 1984, pp. 133-140). Much of the soil does not reach the bottom, but disperses throughout the water, adversely affecting the biota's environment. In fact, when the depth of the disposal site is greater than 400 feet, virtually none of the soil reaches the bottom.
In the case of upland disposal, the soil may be transported via pipeline as a watery pulp. The concentration of the pulp is very low; the ratio by weight between the soil and water is from 1/7 to 1/15, or one part of soil to every 7 to 15 parts of water. At the disposal site, the soil is separated from the water, and the water is returned to the sea or river as an effluent, or discharge. Because some soil remains in the returned water, pollution of the water environment is very likely. Calculations have shown that every 500 cubic m. of soil dredged can potentially pollute up to 40 sq. km. of sea. In addition, the volume and types of contaminants originally in the soil and water affect the extent of the environmental damage.
To alleviate and even altogether avoid the above problems, the soil transportation and disposal operations must be removed from the dredging process.